Human skeletal muscle strength is defined as the maximal force generated by a voluntary contraction (MVC), and is largely determined by muscle 1) size, and 2) activation. Muscle size may be quantitated by its fat-free cross-sectional area (CSA), obtained using magnetic resonance imaging (MRI). The relationship between muscle strength and size is linear and is expressed as specific strength (MVC/CSA). Activation (defined as the process by which the signal to contract is transmitted from the CNS to the muscle's contractile machinery) can be quantitated using noninvasive neurophysiologic techniques. Function (e.g., physical activity level) may modulate both the size and activation of skeletal muscle. It has recently been suggested that estrogen replacement therapy (ERT) preserves or enhances muscle strength in post-menopausal women. The possible mechanism(s) of this preservation of strength is not known. Clinically, it would be useful to know whether long-term use of ERT influences the development of sarcopenia in elderly women. It is also important for the design of future studies of muscle function in elderly women to know whether ERT must be accounted for. The Specific Aims of this exploratory project are: Specific Aim #1: To compare muscle strength, CSA, specific strength and activation between ERT and control women. Plans are to investigate the effects of long-term ERT on dorsiflexor muscle function by comparing dorsiflexor muscle isometric strength (MVC, Newtons), fat-free cross-sectional area (cm2), specific strength and activation in 25 women on (5+) years and 25 women not on (in previous 5 years) ERT. All women will be 65-85 yrs old, 10+ yrs post-menopause. Because of the potential influence of activity level on the measures, the investigators will select women with similar levels of physical activity. They will test the following preliminary hypotheses: Hypothesis#1: Muscle strength and CSA will be higher in ERT compared to control women. Hypothesis #2: Muscle activation will be similar in ERT compared to control women. Specific Aim #2: To compare muscle composition and the influence of physical activity level in ERT and control women. The investigators will separately quantitate the CSA of the muscle and adipose tissue compartments of the dorsiflexor muscles in the same ERT and control groups. They will then test Hypotheses #3A: Intramuscular fat content will be lower in ERT compared to control and #3B: Intramuscular fat content will be related to physical activity level, regardless of estrogen status.